Track progress across work sessions using SESSION.md with git checkpoints and concrete next actions.

• Converts IMPLEMENTATION_PHASES.md into a living SESSION.md document that maps phases to sessions, tracks current stage (Implementation/Verification/Debugging), and maintains concrete "Next Action" pointers with file locations and line numbers
• Provides /wrap-session command to update SESSION.md, create git checkpoints, and output summaries; /continue-session command to load context, show git history, and resume from the last action
• Distinguishes phases (units of work with exit criteria) from sessions (units of context), allowing multi-session phases and mid-context checkpoints without losing progress
• Uses emoji status icons (⏸️ pending, 🔄 in progress, ✅ complete, 🚫 blocked) and structured checkpoint commits to enable reliable context handoff and phase transitions